1. Field of the Invention
The present invention relates to an input/output control circuit for an IC card equipped with one-time PROM-ICs.
2. Description of the Related Art
FIG. 3 shows a conventional IC card equipped with, one-time PROM-ICs. A card enable signal line 5 is connected to the chip enable signal input terminals CE of a plurality of one-time PROM-ICs 1 through an address decoder 2. An address buffer 3 is connected to each of the one-time PROM-ICs 1 through an internal address bus 7, and an address bus 6 is connected to the address buffer 3. An output enable signal line 10 and a program control signal line 11 are connected to the output enable signal input terminal OE and the program control signal input terminal PGM, respectively, of each of the one-time PROM-ICs 1 through a control signal control circuit 4. Upper address signal lines 8 are connected to the address decoder 2 and the control signal control circuit 4. A data bus 9 and a power supply line 12 are also connected to each of the one-time PROM-ICs 1.
In a data reading operation, an address is supplied to the address bus 6 and the upper address signal lines 8. The card enable signal line 5 and the output enable signal line 10 are brought to L level, and the program control signal line 11 is brought to H level, On the basis of the address specified on the upper address signal lines 8, one of the plurality of PROM-ICs 1 is selected by the address decoder 2 and the control signal control circuit 4, and a chip enable signal at the L level is input to the chip enable signal input terminal of the selected IC 1 from the address decoder 2. At the same time, an output enable signal at the L level and a program control signal at the H level are input to the output enable signal input terminal OE and the program control signal input terminal PGM, respectively, of the selected IC 1 from the control signal control circuit 4. As a result, the selected IC 1 assumes a read state, and the data at the address specified on the address bus 6 is output to the data bus 9.
On the other hand, in a data writing operation, a program voltage Vpp of 12.5 V is supplied to each of the ICs 1 through the power supply 12, an address is given to the address bus 6 and the upper address signal lines 8, and write data is given to the data bus 9. In this state, when the card enable signal line 5 and the program control signal line 11 are brought to the L level, and when the output enable signal line 10 is brought to the H level, a chip enable signal and a program control signal at the L level and the output enable signal at the H level are input to the one-time PROM-IC 1 selected on the basis of the address on the upper address signal lines 8. As a result, the selected IC 1 assumes the write state, and the write data on the data bus 9 is written at the address specified by the address bus 6.
However, as shown in FIG. 3, the conventional IC card equipped with one-time PROM-ICs has no output buffer, and thus has a problem with respect to resisting static electricity. The conventional IC card also has the problem that the output terminal capacity and the electrical properties thereof change with a change in the number of the one-time PROM-ICs mounted thereon.